1. Field of the Invention
The present invention relates to an optical receiver, and more particularly to an optical receiver that can be applied to waveform shaping for decoded light in optical code division multiplexing (OCDM), for example.
2. Description of Related Art
In recent years, OCDM is receiving attention as a multiplexing method suitable for increasing the speed and capacity of optical metro access networks. OCDM is a method that realizes multiplexing by coding/decoding channels with the aid of optical orthogonal codes in an optical receiver. Examples of coding/decoding methods include direct spreading methods, time spreading methods, wavelength hopping methods, and time spreading/wavelength hopping methods.
First, the coding and decoding steps in the time spreading/wavelength hopping method cited in Japanese Patent Application Laid-open No. 2000-209186 is described with reference to FIGS. 2(A) to 4(E). In order to show a signal propagated in a transmission line, the diagrams of the pulse signal that are depicted with respect to the time axis at positions indicated by the arrows drawn out from the circles in FIGS. 2(A), 3(A), and 4(A), with the circles drawn around the transmission line in the figures, are expressed as charts that show the time axis waveform of the signal propagated in the transmission line.
Transmission data 201, which is an optical signal, is fed to a coder 203 on the transmission side, as shown in FIG. 2(A). The transmission data 201 (202), which is an optical signal, is data in which light composed of wavelengths λ1 to λ3 with a predetermined number of wavelengths (three wavelengths in FIGS. 2(A) to 4(D)) is modulated for intensity in a return-to-zero (RZ) format in accordance with the transmission data, which is an electric signal, and valid data is generated in a time slot (chip) for each data cycle, as shown in FIG. 2(B). The wavelength components contained in the transmission data 201 are delayed (coded) in the coder 203 by a specific interval of time in accordance with a specific coding pattern (Code 1), resulting in an optical signal 205 that has a waveform spread out along the time axis, as shown in FIG. 2(C).
Thus, the optical signal 205 obtained by time spreading with a delay time that corresponds to the wavelength components arrives at the decoder 206 by way of the transmission line 204.
In the decoder 206, the wavelength components in the inputted optical signal 205 are delayed (decoded) by a specific interval of time in accordance with the specific coding pattern (Code 1) and de-spread along the time axis (the differences in delay time of the wavelength components are offset), as shown in FIG. 2(D), the wavelength components are superimposed in the same chip period, and received data 207 (208) that is the same as the initial transmission data 201 (202) is obtained. The waveform of the received data 207 (208) obtained when the coding patterns of such coders 203 and decoders 206 match each other is referred to as an autocorrelation waveform.
A case in which the coding patterns in the coder-203 and the decoder 206 differ from each other is shown in FIGS. 3(A) to 3(D). In other words, in this case the coder 203 has a coding pattern (Code 2), and the decoder 206 has a decoding pattern (Code 1).
When different codes are used in the coder 203 and the decoder 206, as shown in FIGS. 3(A) to 3(D), the time delay difference is not offset by the processing in the decoder 206, and the waveform (received data) has a low peak and is spread out in the direction of the time axis, as shown in FIG. 3(D). The waveform of the received data obtained when the coding patterns of the coder and decoder are different in this manner is referred to as a cross-correlation waveform.
A case in which a coded optical signal is multiplexed is described with reference to FIGS. 4(A) to 4(E). FIG. 4(A) is a diagram that shows a case in which signals respectively coded in two different coders (Code 1) and (Code 2) are merged, propagated through a transmission line, and decoded in a decoder (Code 1). The transmission data respectively coded in the coders (Code 1) and (Code 2) have optical pulse waveforms on the time axis shown in FIGS. 4(B) and 4(C). In the optical pulses, light signals with the wavelengths λ1 to λ3 are interspersed and overlain on the time axis, as shown in FIGS. 4(B) and 4(C). Pieces of transmission data respectively coded in the coders (Code 1) and (Code 2) are merged, propagated in the transmission line, fed to the decoder (Code 1), and decoded at that point. The time waveform of the optical pulse signal propagating through the transmission line has the shape shown in FIG. 4(D). The time waveform of the received data (decoded signal) that is outputted by the decoder has the shape shown in FIG. 4(E), signifying that received data is obtained as a sum of the autocorrelation waveform and the cross-correlation waveform.
The sum of the autocorrelation waveform and the cross-correlation waveform in the received data (decoded signal) is obtained in the same manner as with any coding/decoding method other than a time spreading/wavelength hopping method. The cross-correlation waveform becomes noise with respect to the desired signal, thereby causing the signal-to-noise ratio (SN ratio) during data identification to degrade.
Consequently, a method whereby a time gate is applied during the optical signal stage to eliminate cross-correlation waveforms has already been proposed as a method for improving the SN ratio. (Refer to a publication, for example, K. Kitayama, et al., “Optical Code Division Multiplexing (OCDM) and Its Applications to Photonic Networks,” IEICE Trans. Fundamentals, Vol. E82-A, No. 12, pp. 2616-2625, December 1999).
Elimination of cross-correlation waveforms with the aid of a time gate is briefly described with reference to FIGS. 5(A) to 5(D). In order to show a signal propagated in a transmission line, the diagrams of the pulse signal that are depicted with respect to the time axis at positions indicated by the arrows drawn out from the circles in FIG. 5(A), with the circles drawn around the transmission line in the figures, are expressed as charts that show the time axis waveform of the signal propagated in the transmission line. The time waveform 304 represented by the sum of the autocorrelation waveform (desired signal) indicated by the white rectangles and the cross-correlation waveform (noise) indicated by the shaded rectangles, as shown in FIG. 5(B), is the optical signal 301 after decoding. In the time gate 302, the time waveform is processed by the time gate signal 305 shown in FIG. 5(D) such that the signal is allowed to pass (gate on) at the same time as the peak of the autocorrelation wave, and the gate is blocked (gate off) at other times. At this time, an optical signal 306 with noise removed is obtained because only the desired signal passes through the time gate 302.
In the above-noted publication (Kitayama), a decoded optical signal is split into two, an optical clock is extracted from one of the split optical signals with the aid of a mode-locking laser, the state of polarization of the optical clock and the other decoded optical signal are controlled, and four-wave mixing is then generated in a semiconductor amplifier to realize a time gate. After having passed through the time gate, the optical signal has unneeded wavelength components due to the four-wave mixing, so the desired signal alone is extracted by means of a wavelength filter.
When eliminating noise with the above-described time gate, the peak of the autocorrelation wave and the gate-on timing must be matched to allow passage through the gate. Consequently, the timing of the received signal must be extracted in the receiver in an actual OCDM system. However, the timing of the desired signal must be extracted from a coded multiplexed signal in which the noise level is changing. Furthermore, the signal is transmitted through optical fiber that is affected by temperature fluctuations and other environmental factors, so the timing of the signal pulse string differs on the transmission and receiving sides, and is constantly changing with time. The timing must be extracted under such, conditions.
The method disclosed in the above-described publication (Kitayama) is suitable for increasing speed because it is a method that uses a time gate based on optical signal processing.
However, a large number of optical elements are required to realize a time gate, and the system is made more expensive as a result. Also, the polarization state of the optical clock and the decoded optical signal are fixed in a desired shape, so adjustments must be made with a polarization state control device in addition to many aspects that require adjustment, and adjustment work is complicated.
A need therefore exists for an optical receiver which has a simple, low-cost configuration, in which a time gate can be realized, and which has few aspects that require adjustment.